Background is a simple iPhone app available through Cydia for free. It simply allows you to run your apps in background mode. To run applications on background just press the home button for two seconds and then a screen arrives in the center saying that the application is running on background. To quit the app continue holding the home button for few more seconds until the app closes and brings you to the main screen. Its good for apps like fring and IM etc which requires constant access.
It is similar to insomnia available on iPhone 2.0 through Cydia. For more details contact the developer’s site
http://code.google.com/p/iphone-backgrounder/wiki/Documentation
Update: A new version of Backgrounder for iPhone 3.0 is now available. Though it has less features compared to the version available for 2.x